Lateral spread of photon

[LaserMind]

When a photon is at a large distance fom it's starting position then the wave is spread-out laterally.

How can a wavefront for one photon collapse instantly over a massive surface area?

 

[bhobba]

How can a wavefront for one photon collapse instantly over a massive surface area?

Well leaving aside the issue of if a photon has an actual position (it doesnt) the wave function is not real in any kind of physical sense - it simply an aid to calculating where the photon will be detected. That such resides over a massive area simply indicates the area it can be detected is large.

Thanks
Bill

 

[LaserMind]

But that means that the whole area of the wavefront needs to know instantly that it's been detected.

 

[Khashishi]

This is just another question about the meaning of the collapse of a wavefunction. This question isn't settled yet, so just choose an interpretation that you like.
If you don't know anything about interpretations of quantum mechanics, then just don't worry about it. The wavefunction is just some vector in some equations. Nothing happens when it collapses, since it's just a representation in your head that changes.

 

[Khashishi]

On the other hand, if you are talking about the classical electromagnetic field, I don't think that is defined for one photon. In order to have a classical electromagnetic field, you don't have a well-defined photon count, but rather some expected photon rate.

edit: I am probably wrong. I'm looking at M.G. Raymor 2005 http://arxiv.org/ftp/quant-ph/papers/0604/0604169.pdf which says that the electromagnetic field based off Maxwell's equations is indeed the same thing as the quantum wavefunction for a single photon.

So in the end, it looks like the electromagnetic field is no more or less "real" than the quantum wavefunction.

 

[bhobba]

But that means that the whole area of the wavefront needs to know instantly that it's been detected.

Why does a function whose square gives the probability of detecting something need to know anything?

Thanks
Bill

 

[bhobba]

.So in the end, it looks like the electromagnetic field is no more or less "real" than the quantum wavefunction.

Classically the EM field is considered real like a classical particle is real. The reason has to do with no-go theorems worked out by Wigner - you can find some of the detail in Ohanion - Gravitation And Space-Time. In order for momentum and energy to be conserved, and Noethers Theorem says you can't really ignore the issue, a holder of momentum and energy is needed - that is the field. Physicists, classically, usually consider momentum and energy very real, hence the field is very real - classically.

Quantum mechanically of course the issue is much more nebulous - but classically it's clear.

Also intermixing classical and quantum in EM is fraught with danger - that really requires QFT - and things become even murkier eg its described by a Fock space and photon position is not an observable.

Thanks
Bill